1. Field of the Invention
The present invention relates to a transparent conductive film low in reflectance and resistance, having a double-layer structure comprising a lower layer containing a fine metal powder and a silica-based upper layer and a composition for forming a transparent conductive film, suitable for forming the lower layer film described above. The transparent conductive film of the invention is suitable for imparting functions such as prevention of electrification, shielding of electromagnetic wave, and anti-dazzling property (prevention of disturbing reflection) to a transparent substrate such as a cathode ray tube (CRT) and an image display section of various display units.
2. Discussion of the Related Art
Glass composing an image display section (screen) of various display units such as a cathode ray tube (CRT for TV or display), a plasma display, an electroluminescence (EL) display, and a liquid crystal display is easily susceptible to deposition of dust on the surface under the electrostatic effect, and the insufficient anti-dazzling property leads to a problem of an unclear image as a result of external light or reflection of an external image. More recently, people are worrying about possible adverse effect of electromagnetic waves emitted from a cathode ray tube on human health and accordingly countries are enacting standards for low-frequency leaking electromagnetic waves.
As measures against deposition of dust or leakage of electromagnetic waves, it is possible to adopt means for forming a transparent conductive film or the outer surface of screen because of the electrification preventing effect or electromagnetic waves. It has been the conventional practice for imparting anti-dazzling property to apply a non-glare treatment of causing light scattering by providing fine irregularities to the screen glass surface with the use of hydrofluoric acid or the like. The non-glare treatment poses problems such as a lower resolution of the image and a decreased visibility.
Attempts have been made to impart functions of preventing electrification (preventing dust from depositing) and preventing reflection by means of a double-layer film having a transparent conductive film having a high refractive index and a transparent overcoat film having a low refractive index formed thereon. With such a double-layer film, particularly when there is a large difference in refractive index between the high-refractivity film and the low-refractivity film, the reflected light from the surface of the low-refractivity film, which is the upper layer, is offset by the interference of the reflected light from the interface with the high-refractivity film which is the lower layer, thus resulting in an improved anti-dazzling property.
When the transparent conductive film has a high electric conductivity, an electromagnetic wave shielding effect is also available.
For example, Japanese Unexamined Patent Publication No. 5-290,634 discloses a double-layer film having a reflectance reduced to 0.7% by a process comprising the steps of coating an alcoholic dispersed solution in which a fine Sb-doped tin oxide (ATO) powder is dispersed by the use of a surfactant onto a glass substrate, forming a conductive film having a high refractive index by drying the resultant film and forming thereon a silica-based low refractive film formed from alkoxysilane which may contain magnesium fluoride.
Japanese Unexamined Patent Publication No. 6-12,920 discloses findings that a low reflectance is available by causing a high-refractivity layer and a low-refractivity layer formed on a substrate to have an optical film thickness nd (n: film thickness, d: refractive index) of xc2xdxcex and xc2xcxcex (xcex=wavelength of incident light), respectively. According to this patent publication, the high-refractivity layer is a silica-based film containing a fine ATO or Sn-doped indium oxide (ITO) powder and the low-refractivity film is a silica film.
Japanese Unexamined Patent Publication No. 6-234,552 discloses also a double-layer film comprising an ITO-containing silicate high-refractivity conductive film and a silicate glass low-refractivity film.
Japanese Unexamined Patent Publication No. 5-107,403 discloses a double-layer film comprising a high- refractivity conductive film formed by coating a solution containing a fine conductive powder and Ti salt and a low-refractivity film.
Japanese Unexamined Patent Publication No. 6-344,489 discloses a blackish double-layer film comprising a first high-refractivity film consisting of a fine ATO powder, a black conductive fine powder (preferably, carbon black fine powder) in which solids are densely passed and a silica-based low-refractivity film formed thereon.
With a transparent conductive film using a semiconductor-type conductive powder such as ATO or ITO, however, it is usually difficult to achieve a lower resistance so as to give an electromagnetic wave shielding effect and even if it is possible to achieve a lower resistance, leads to a seriously decreased transparency. Particularly now that regulations on leaking electromagnetic waves from a CRT are becoming more strict than ever, it is difficult to cope with such circumstances with the foregoing conventional art because of an insufficient electromagnetic wave shielding effect and, as a result, there is an increasing demand for a transparent conductive film having a lower resistance and bringing about a more remarkable electromagnetic wave shielding effect.
Adoption of a vapor depositing process such as sputtering permits formation of a transparent conductive film having a high electromagnetic wave shielding effect but this technique cannot easily be adopted for a mass-produced product such as TV sets from cost consideration.
The present invention has, therefore, an object to provide a double-layer structured transparent conductive film having a low reflectivity, which has a low resistance so as to display an electromagnetic wave shielding effect on a high level, while maintaining a transparency and a low haze value so as not to impair visible identification of a CRT, and can impart an anti-dazzling function useful for preventing reflection of an external image.
Another object of the invention is to provide a transparent conductive film provided with a high contract property, in addition to the foregoing properties.
A further object of the invention is to provide a transparent conductive film in which the reflected light is not bluish or reddish but is substantially colorless.
A further object of the invention is to provide a transparent conductive layer forming composition excellent in film forming property, containing a fine metal powder, in which film irregularities such as color blurs, radial stripes and spots are alleviated or even eliminated.
A further object of the invention is to provide a transparent conductive film forming composition, excellent in storage stability, containing a fine metal powder.
The present inventors noted that, in view of the recent strict standards for electromagnetic wave shielding property of a CRT, it was desirable to use, not a fine inorganic powder of the semiconductor type such as ATO or ITO, but a fine metal powder having a higher conductivity as a conductive powder used for a transparent conductive film.
The present invention further provides a double-layer structured transparent conductive film having a low reflectance and electromagnetic wave shielding property, comprising a lower layer containing a fine metal powder in a silica-based matrix provided on the surface of a transparent substrate, and a silica-based upper layer provided thereon.
The lower layer containing the fine metal powder may contain a black powder (for example, titanium black) in addition to the fine metal powder. This improves contrast of the transparent conductive film.
In the lower layer, secondary particles of the fine metal powder may be distributed so as to form a two-dimensional net structure having pores not containing therein a fine metal powder. This enables a visible light to pass through the pores in the net structure, thus, considerably improving transparency of the transparent conductive film.
Further, the lower layer has concave and convex portions on the surface thereof. The lower layer convex portions have an average film thickness within a range of from 50 to 150 nm, and the concave portions have an average thickness within a range of from 50 to 85% of that of the convex portions. The convex portions may have an average pitch within a range of from 20 to 300 nm. This leads to a flat reflection spectrum from the transparent conductive film, resulting in substantially a colorless reflected light.
Accordingly, the present invention provides a composition forming a conductive film containing a fine metal powder suitable for use for the formation of the lower layer.
In an embodiment, the conductive film forming composition comprises a dispersed solution formed by dispersing a fine metal powder having a primary particle size of up to 20 nm in an amount within a range of from 0.20 to 0.50 wt. % in an organic solvent containing water. The solvent contains (1) a fluorine-containing surfactant in an amount within a range of from 0.0020 to 0.080 wt. %, and/or (2) a polyhydric alcohol, polyalkyleneglycol and monoalkylether derivative in a total amount within a range of from 0.10 to 3.0 wt. %. It is possible to form from this composition a conductive film excellent in film forming property in which film irregularities such as color blurs, radial stripes or spots are alleviated or even eliminated.
In another embodiment, the composition comprises an aqueous dispersed solution containing a fine metal powder having a primary particle size of up to 20 nm in an amount within a range of from 2.0 to 10.0 wt. %, with an electric conductivity of up to 7.0 mS/cm of the dispersant and a pH within a range of from 3.8 to 9.0. There is, thus, provided a conductive film forming composition containing a fine metal powder, excellent in storage stability, used by diluting with a solvent.